1. Field of the Invention
The present invention relates to an oxide superconducting wire having a sheath of high strength and/or high resistance and a method of preparing the same, and more particularly, it relates to a technique for attaining improvement in critical current density of a bismuth based oxide superconducting wire and providing a wire which is advantageously applied to a high magnetic field coil, a current lead, or ac application.
2. Description of the Background Art
In recent years, oxide superconducting materials are watched with interest as superconducting materials exhibiting higher critical temperatures. For example, it is known that bismuth based oxide superconducting materials include those having critical temperatures of 110 K and 80 K, as phases exhibiting high critical temperatures. It is also known that the 110 K phase has a 2223 composition in a Bixe2x80x94Srxe2x80x94Caxe2x80x94Cu system or a Bixe2x80x94Pbxe2x80x94Srxe2x80x94Caxe2x80x94Cu system partially replacing Bi with Pb, while the 80 K phase has a 2212 composition in a similar system.
In relation to a method of preparing an oxide superconductor, there is a method of charging powder of an oxide superconductor or raw material therefor into a metal sheath and performing plastic working and a heat treatment thereto. Due to this process, the powder which is charged in the metal sheath is sintered to form a superconductor. This method is called a powder-in-tube method, and advantageously applied to preparation of a long superconducting wire, for example. The obtained wire can be applied to a power cable or a coil.
In the powder-in-tube method, the sheath which is charged with the powder is subjected to plastic working such as wire drawing and rolling. When the sheath is prepared from pure silver, ideal compressive force cannot be applied to the powder in single plastic working, due to the relatively low strength of silver. When a wire prepared from the sheath of pure silver is heat treated for sintering the oxide superconductor, a temperature which is higher than the softening temperature of silver is employed and hence the strength of silver is reduced after the sintering. Therefore, excessive distortion is readily applied when the obtained wire is handled or coiled, leading to deterioration of superconductivity such as the critical current density.
Japanese Patent Laying-Open No. 2-8335 (1990) discloses a sheath for preparing an oxide superconducting wire which consists of a pipe of an Ag alloy containing 1 to 10 at. % of Mn. Such an alloy is higher in hardness than Ag. This gazette describes that the sheath which is formed by the pipe of an Ag alloy containing 1 to 10 at. % of Mn can be reduced in thickness as compared with an Ag sheath, and hence wire drawing is simplified and oxygen deficiency of the oxide superconductor can be readily recovered. The gazette also describes that the sheath is suitable as that for a drawing method of applying intermediate annealing of 100 to 300xc2x0 C. every time sectional compressibility reaches 2 to 50. However, the sheath containing Mn in such high concentration is unsuitable for a process requiring a heat treatment at a high temperature of 840 to 850xc2x0 C., due to extreme reaction with the superconductor.
Summary of General Lectures at Autumn Meeting of the Japan Institute of Metals, October 1987, p. 236 also discloses sheath materials of Agxe2x80x942 at. % Mn and Agxe2x80x945 at. % Mn in the powder-in-tube method for a Yxe2x80x94Baxe2x80x94Cuxe2x80x94O system. However, these sheaths are also unsuitable for the heat treatment at 840 to 850xc2x0 C. Particularly in preparation of bismuth based oxide superconducting wires, it is difficult to attain high critical current densities with these sheaths.
When the wire employing an oxide superconductor is applied to a current lead of a superconducting magnet employing liquid helium as a refrigerant, it is desirable to sufficiently suppress the amount of heat penetration through the current lead. However, the wire which is formed by the silver sheath has a large amount of heat penetration through silver having high heat conductivity, and improvement of this point is awaited.
When the superconducting wire is energized with a direct current which is lower than its critical current density, substantially no loss is caused. When the superconducting wire is energized with an alternating current, however, loss is caused even if the current is below the critical current density. Particularly in the silver sheath wire, it is necessary to reduce eddy current loss and coupling loss which is caused in the sheath portion.
In order to solve the aforementioned problems in the oxide superconducting wire which is formed by the powder-in-tube method, an object of the present invention is to provide a wire having the following characteristics:
(1) The oxide superconductor is held in a sheath at a higher density.
(2) The wire exhibits a high critical current density (Jc).
(3) The superconducting property such as the critical current density is hardly reduced by bending strain, stress in handling, hoop stress in application to a coil, and the like.
(4) The wire has a small amount of heat penetration upon application to a current lead.
(5) The wire has small ac loss.
The wire according to the present invention is an oxide superconducting wire obtained by performing plastic working and sintering on a sheath, consisting essentially of a stabilizing metal, which is charged with raw material powder capable of forming an oxide superconductor by sintering, and comprises a filament consisting essentially of the oxide superconductor and the stabilizing metal covering the same. In this wire, the stabilizing metal includes such a silver alloy that at least either mechanical strength or specific resistance is higher than that of silver. This wire has a critical temperature of at least 100 K and a critical current density (at 77 K and 0 T) of at least 104 A/cm2, with a yield point of at least 5 kg/mm2 at the room temperature in a tensile test.
According to the present invention, the wire has the critical temperature of at least 100 K, preferably at least 105 K, and the critical current density (at 77 K and 0 T) of at least 104 A/cm2, preferably at least 20,000 A/cm2, with the yield point of at least 5 kg/mm2, preferably at least 10 kg/mm2, at the room temperature in the tensile test. When the sheath of the stabilizing. metal includes that of an Agxe2x80x94Mn alloy containing at least 0.01 at. % and less than 1 at. % of Mn and/or an Agxe2x80x94Auxe2x80x94Mn alloy containing at least 1 at. % and not more than 30 at. % of Au and at least 0.01 at. % and less than 1 at. % of Mn, for example, the wire according to the present invention can be provided with specific resistance of at least 0.60 xcexcxcexa9xc2x7cm at a temperature immediately above the critical temperature. The specific resistance of the wire can be increased also when a silver alloy containing a metal selected from the group consisting of Sb, Pb and Bi is employed in accordance with the present invention. According to the present invention, the specific resistance at the temperature immediately above the critical temperature can be defined as specific resistance at such a temperature that electric resistance deviates from the linear region and abruptly starts to reduce when temperature change of the electric resistance is measured, while the yield point at the room temperature in the tensile test can readily be defined since an obvious yield phenomenon is recognized in the tensile test. Ac loss and the amount of heat penetration in the case of employing the inventive wire as a current lead can be reduced by improving the specific resistance at the temperature immediately above the critical temperature. If the yield point in the tensile test is less than 5 kg/mm2, however, the density of the oxide superconductor is not remarkably improved in the sheath, and improvement of the mechanical strength against bending distortion and stress in handling is not remarkable either. The silver alloy employed for the stabilizing metal sheath for improving the mechanical strength and/or the specific resistance is described later in detail.
In another aspect of the present invention, the oxide superconducting wire according to the present invention is obtained by performing plastic working and sintering on a sheath, consisting essentially of a stabilizing metal, which is charged with raw material powder capable of forming an oxide superconductor by sintering, and comprises a filament consisting essentially of the oxide superconductor and the stabilizing metal covering the same. The stabilizing metal includes such a silver alloy that at least either mechanical strength or specific resistance is higher than that of silver. Further, this stabilizing metal comprises a first portion directly covering the filament and a second portion covering the first portion, and the first portion is adapted to prevent the component of the second portion from diffusing into and reacting with the oxide superconductor. Due to such a structure, a wire having a critical temperature of at least 100 K and a critical current density (at 77 K and 0 T) of at least 104 A/cm2 with a yield point of at least 5 kg/mm2 at the room temperature in a tensile test can be provided.
In still another aspect, the oxide superconducting wire according to the present invention is an oxide superconducting wire obtained by performing plastic working and sintering on a sheath, consisting essentially of a stabilizing metal, which is charged with raw material powder capable of forming an oxide superconductor by sintering, and comprises a plurality of filaments consisting essentially of the oxide superconductor and the stabilizing metal covering the same. The stabilizing metal includes a silver alloy having higher mechanical strength and specific resistance than silver, and comprises a first portion directly covering the filaments and a second portion covering the first portion, while the first portion consists essentially of a silver alloy having higher specific resistance than silver for reducing ac loss which is caused across the plurality of filaments. Due to such a structure, a wire having a critical temperature of at least 100 K and a critical current density (at 77 K and 0 T) of at least 104 A/cm2 with a yield point of at least 5 kg/mm2 at the room temperature in a tensile test can be provided.
A method of preparing an oxide superconducting wire is provided according to the present invention. This method comprises a step of performing plastic working and sintering on a stabilizing metal sheath which is charged with raw material powder capable of forming an oxide superconductor by sintering. In such a method, the stabilizing metal sheath is prepared from a silver alloy which is selected from the group consisting of the following alloys (a) to (h). When the stabilizing metal sheath is prepared from a silver alloy containing Mn in this method, the sintering step is preferably carried out under an atmosphere containing oxygen at a partial pressure of at least 0.01 atm. at a temperature of not more than 850xc2x0 C. If the stabilizing metal sheath is prepared from a silver alloy containing a metal selected from the group consisting of Sb, Pb and Bi, on the other hand, the sintering step is preferably carried out under an atmosphere containing oxygen at a partial pressure of not more than 0.08 atm. at a temperature of not more than 850xc2x0 C.
(a) An Agxe2x80x94Mn alloy containing at least 0.01 at. % and less than 1 at. % of Mn;
(b) an Agxe2x80x94Auxe2x80x94Mn alloy containing at least 1 at. % and not more than 30 at. % of Au and at least 0.01 at. % and less than 1 at. % of Mn;
(c) an Agxe2x80x94Sb alloy containing at least 0.01 at. % and not more than 5 at. % of Sb;
(d) an Agxe2x80x94Auxe2x80x94Sb alloy containing at least 1 at. % and not more than 30 at. % of Au and at least 0.01 at. % and not more than 5 at. % of Sb;
(e) an Agxe2x80x94Pb alloy containing at least 0.01 at. % and not more than 3 at. % of Pb;
(f) an Agxe2x80x94Auxe2x80x94Pb alloy containing at least 1 at. % and not more than 30 at. % of Au and at least 0.01 at. % and not more than 3 at. % of Pb;
(g) an Agxe2x80x94Auxe2x80x94Bi alloy containing at least 1 at. % and not more than 30 at. % of Au and at least 0.01 at. % and not more than 3 at. % of Bi; and
(h) an Agxe2x80x94Bi alloy containing at least 0.01 at. % and not more than 3 at. % of Bi.
Another method of preparing a multifilamentary oxide superconducting wire is provided according to the present invention. This method comprises the steps of charging a first stabilizing metal sheath with raw material powder capable of forming an oxide superconductor by sintering and performing plastic working thereon for obtaining a strand, engaging a plurality of such strands in a second stabilizing metal sheath and then performing plastic working thereon for obtaining a multifilamentary wire, and heat treating the multifilamentary wire for obtaining a sintered body of the oxide superconductor. According to this method, the first stabilizing metal sheath is prepared from a material which is selected from the group consisting of Ag, an Agxe2x80x94Zr alloy, an Agxe2x80x94Ti alloy, an Agxe2x80x94Au alloy and the aforementioned alloys (a) to (h). On the other hand, the second stabilizing metal sheath is prepared from a material which is selected from the group consisting of Ag, an Agxe2x80x94Mg alloy, an Agxe2x80x94Ni alloy, an Agxe2x80x94Mgxe2x80x94Ni alloy, an Agxe2x80x94Zr alloy and the aforementioned alloys (a) to (h). At least either the first or second stabilizing metal sheath is prepared from a silver alloy selected from the aforementioned groups. Therefore, not both of the first and second stabilizing metal sheaths consist essentially of silver. When either stabilizing metal sheath is prepared from a silver alloy containing Mn in this method, the heat treatment step is preferably carried out under an atmosphere containing oxygen at a partial pressure of at least 0.01 atm. at a temperature of not more than 850xc2x0 C. When either stabilizing metal sheath is prepared from a silver alloy containing a metal selected from the group consisting of Sb, Pb and Bi, on the other hand, the heat treatment step is preferably carried out under an atmosphere containing oxygen at a partial pressure of not more than 0.08 atm. at a temperature of not more than 850xc2x0 C. Additionally, the second stabilizing metal sheath may be prepared from an Agxe2x80x94Mn alloy containing Mn in the range of 1 at. % to 5 at. %.